The Twilight Saga: Eclipse is like some broodingly repressed wide-screen youth-pop melodrama from the 1950s — and I mean that as a compliment. The story, at heart, is earnest and humorless teen romantic glop, but its feelings aren’t fake, and the movie is compulsively watchable; it has a passionflower intensity. In case you haven’t been keeping up, the plot — and the blood — have thickened a lot since the first Twilight film two years ago. In that initial, relatively uncomplicated chapter, the ever-so-mildly dark and alienated Bella Swan (Kristen Stewart) fell for the dashing vampire Edward Cullen (Robert Pattinson) because of his pasty-faced erotic appeal (sideburns, James Dean hair, golden eyes, eternal stricken look), but also because he offered himself up as the ultimate protector: swoony and chivalrous, a spangly-skinned knight of the woods who vowed to guard her innocence.

In Eclipse, adapted from the third novel in Stephenie Meyer’s girl-power-meets-retro-Harlequin fantasy series, Bella now needs to be saved from no one so much as herself. She loves Edward so completely that her fears about his “otherness” have been all but dissolved. She can’t wait to go over to the undead side, and to lose her virginity, too — though Edward, ever the gentleman, has other ideas. He’ll agree to make her a vampire only if she marries him (he seems to propose about every third scene). The more idealized their love becomes, the more it attracts danger like the storm clouds roiling over the Pacific Northwest.

In Seattle, an army of fresh, hungry vampires — ”newborns” — has gone on a rampage. Since they’ve only just recently been ”converted” and still have remnants of human blood, they’re more ruthless and insatiable than older vamps; their genesis may be revenge, but we can sense its ultimate effect — to destroy Bella and Edward. The other threat to the couple’s love comes from Jacob Black, the snub-nosed Quileute Indian werewolf played with quick responses and a rock-steady gaze by Taylor Lautner, who’s like the young Matt Damon in the body of Marky Mark. Prior to Eclipse, Jacob nurtured his crush on Bella mostly from afar, but now he emerges as a furious, literally hot-blooded rival for her affection. When she insists that she’s with Edward, he narrows those dagger eyebrows and basically says: You love me — you just don’t know it yet. Which is a little scary. In Eclipse, Edward and Jacob square off like two king jocks, each devoted to making Bella his prize. Confronted with this much ardent male protection, a girl could die.

As always, Stephenie Meyer is not the most seamless of storytellers. She sets up soap opera horror-movie situations that stutter and ramble forward, muffled by those declarations of adolescent devotion and by Bella’s own tentative, hemming-and-hawing nature. The movie, directed by David Slade (Hard Candy) from a screenplay by Melissa Rosenberg, attempts to turn the passive Bella into more of an aggressor, the brave one who yearns to dissolve Edward’s caution. Yet there are moments when Kristen Stewart’s acting gets a little too moody and remote. That said, she sulks and pines gorgeously, and the film, though it could have used a sharper narrative, coasts along on all that nakedly intense teen emotion. Pattinson, like James Dean, holds back so much that it’s cathartic when he finally lets loose. And the battle between the newborn vamps and a coalition of Edward’s vampire clan and the werewolves is thrillingly shot and edited. It’s not just a war of F/X but a true demon power struggle.

The Twilight movies, like the books on which they’re based, are often mocked. But that’s only because we’re still, on some level, getting used to the novelty of a highly contemporary blockbuster saga that’s this rooted in old-fashioned, borderline masochistic girlish romantic rapture. The movie version of Eclipse, with its dueling boy-monster hunks — a chaste orgy of male gazing — revels in the power that Bella experiences by giving herself over to the powerlessness of love. The movie is about a girl’s primal dream of being desired. That may well be corny, but it’s also an essential antidote to summer-movie hardware. B+